How could I miss this so long? Google Browser Sync keeps Firefox‘ bookmarks, sessions, cookies, tabs and even passwords (if you really want to) in sync. As I constantly switch between multiple computers, this is really a nice thing to have. You need to have a Google account, but as I already use Google Reader and Calendar, I already have one. In order to hide your data from Google (as if this mattered due the amount they are already collecting about us) you can protect the information by a password (PIN).
I’m currently trying it out and up to now I am very pleased with it. One more reason to stick with Firefox